The present invention relates to the portable telephone communications art. Specifically, an active bias network circuit is described which provides a stable reference voltage to a radio frequency amplifier.
Portable telephone communications require a transmitter which is small and which conserves battery power. Additionally, these low power transmitters are subject to both a changing battery power supply voltage, which varies between a high charge level, after being charged, to a lower voltage level when the battery of the telephone discharges. The power amplifier of the telephone transmitter is also subject to thermal overload, due to reflected power from the antenna when it is not impedance matched to the amplifier output stage. Under reflected power conditions, a current is produced from the bipolar transistors of the amplifier which can produce a thermal runaway condition. Additionally, during operation, certain types of modulation, such as CDMA for instance, will produce a baseband modulation component which enters the bias networks for the output amplifier. Under these conditions, the baseband modulation entering the bias network circuit may and be conveyed to other circuits, producing undesirable spurious radiation components in the output power spectrum.
In order to protect the telephone transmitter from these conditions, a bias network must be provided which is isolated from the power amplification stages, and the power amplification stages must be protected against high voltage currents which would produce the thermal runaway condition. Further, these protections must be implemented so that an acceptable current drain is imposed on the battery power supply for the telephone.